iommu/vt-d: Remove commented-out code

These lines of code were commented out when they were first added in commit
ba39592764 ("Intel IOMMU: Intel IOMMU driver"). We do not want to restore
them because the VT-d spec has deprecated the read/write draining hit.

VT-d spec (section 11.4.2):
"
 Hardware implementation with Major Version 2 or higher (VER_REG), always
 performs required drain without software explicitly requesting a drain in
 IOTLB invalidation. This field is deprecated and hardware  will always
 report it as 1 to maintain backward compatibility with software.
"

Remove the code to make the code cleaner.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20230609060514.15154-1-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel/iommu.c

@@ -1312,15 +1312,7 @@ static void __iommu_flush_iotlb(struct intel_iommu *iommu, u16 did,
 			iommu->name, type);
 		return;
 	}
-	/* Note: set drain read/write */
-#if 0
-	/*
-	 * This is probably to be super secure.. Looks like we can
-	 * ignore it without any impact.
-	 */
-	if (cap_read_drain(iommu->cap))
-		val |= DMA_TLB_READ_DRAIN;
-#endif
+
 	if (cap_write_drain(iommu->cap))
 		val |= DMA_TLB_WRITE_DRAIN;